1. Field of the Invention
The present invention relates to a process for producing an information recording medium used as a recording medium of an information processing apparatus and also a process for producing a substrate employed for the medium.
2. Description of the Related Art
One of such information recording mediums is a magnetic disk. The magnetic disk is produced by forming a thin film such as a magnetic layer on a substrate and as the substrate for it, an aluminum or glass substrate has been employed. Reflecting the recent pursuit of high recording densification, a glass substrate which makes it possible to decrease a distance between a magnetic head and a magnetic recording medium has come to be more prevalent compared with an aluminum substrate.
The glass substrate showing such an increasing tendency is, in general, subjected to chemical reinforcement for heightening its strength so that it can endure the impact upon mounting on a magnetic disk driver. The surface of the glass substrate is polished with high accuracy so that the flying height of the magnetic head can be lowered to the utmost, whereby the high recording densification is actualized.
As well as the improvement in the glass substrate, a magnetic head has been changed from a thin-film head to a magnetoresistive type head (MR head) to meet the tendency to high recording densification.
As described above, high evenness on the surface of a magnetic disk is indispensable for the flying height reduction to heighten the recording density. In addition, when an MR head is employed, high flatness on the surface of a magnetic recording medium is also required in order to prevent the generation of TA (thermal asperity). The term "thermal asperity" as used herein means a phenomenon that a projection existing on the surface of a magnetic disk affects an MR head to generate heat, thereby causing fluctuations in the resistance of the head and causing a malfunction in the electromagnetic conversion.
There is an increasing demand for a magnetic disk having high evenness for both the lowering of flying height and prevention of occurrence of thermal asperity. A substrate having a highly even surface is eventually required for a magnetic disk having a surface of high evenness, but the requirement for heightening of the recording density exceeds the level which can be materialized by only polishing the surface of the substrate with high accuracy. In other words, it is impossible to attain high evenness even by polishing with high accuracy if foreign matters adhere onto the substrate. The removal of foreign matters has been carried out conventionally but the conventional tolerance of foreign matters on the substrate is now regarded as inadequate for the recent tendency to heighten the recent densification level.
Examples of such foreign matters include considerably fine iron powders, stainless pieces, glass chips and organic substance which cannot be removed by ordinary washing. When a thin film such as magnetic film is stacked on a glass substrate without removing the particles such as iron powders from the glass substrate, a protrusion is formed on the surface of a magnetic disk, which becomes a cause for disturbing the lowering in the flying height or prevention of thermal asperity.